1. Field of the Invention
The present invention relates to optical storage, specifically, to an optical storage using near field optical effect, to which tracking control function is added, and tracking control method using the optical storage.
2. Discussion of Related Art
An optical recording medium such as a CD-ROM focuses incident rays using an object lens, to form an optical spot. This optical spot is projected on data marks recorded on a recording medium, to read optical information recorded thereon. It is impossible for the optical spot to be smaller than the wavelength of the incident rays because of optical diffraction effect. Accordingly, in case of the incident rays having 780 nm of wavelength, the minimum size of the data mark which is able to be recorded and reproduced becomes 780 nm, reaching the limit of recording density. To overcome the limit, an optical storage using the principle of near field scanning microscope is currently being developed. FIG. 1 shows the configuration of the optical storage using near field optical effect.
FIG. 1 shows a conventional optical storage, FIG. 2 shows the cross section of the fiber probe of FIG. 1, and FIG. 3 shows the relation between data marks recorded on the track of a conventional recording medium and scanning line. Referring to FIG. 1, the conventional optical storage includes a recording medium 6 on which information is recorded, an optical source 7 from which rays of light are projected to recording medium 6, a fiber probe 1 for probing the intensity of radiation, distributed on the recording medium, to read the information recorded on the recording medium, a probe driver 5 for controlling the location of fiber probe 1, and an optical sensor 4 for sensing the intensity of radiation of the rays of light, transmitted through fiber probe 1.
Fiber probe 1, as shown in FIG. 2, is formed of an optical fiber 12 whose one end has conical shape. A metal layer 2 such as Al is formed on the conical portion of the fiber probe, and an aperture 3 is formed at the end of the fiber probe, where metal layer 2 is removed. The aperture conventionally has an opening with the diameter d1 of below 100 nm. Metal layer 2 allows the rays of light from optical source to pass only through aperture 3, and prevents them from being incident to portions other than aperture 3.
The operation principle of the conventional optical storage is described below. When rays of light provided by optical source 7 are projected on recording medium 6, the intensity of radiation is demodulated by data marks 8 recorded on recording medium 6, forming the distribution of intensity of radiation, corresponding to data marks 8, on the front of recording medium 6. Then, fiber probe 1 approaches recording medium 6 under the control of probe driver 5. Here, when the distance between aperture 3 of fiber probe 1 and recording medium 6 is longer than the diameter d1 of aperture 3, the rays of light cannot pass through aperture 3 optically. Accordingly, optical sensor 4 cannot sense any signal.
On the other hand, when fiber probe 1 approaches recording medium more closely, to allow the distance between aperture 3 and recording medium 6 to be shorter than the diameter d1 of aperture 3, a portion of rays of light, which has passed recording medium 6, enters fiber probe 1 through aperture 3, to be applied to optical sensor 4. Optical sensor 4 generates a signal in proportion to the intensity of radiation of the applied light. As described above, when the distance between aperture 3 and recording medium 6 is maintained, probe driver 5 drives fiber probe 1 to scan the surface of recording medium 6. Then, the distribution of intensity of radiation, formed by data marks 8 recorded on recording medium 6, is sensed by optical sensor 4. Optical sensor 4 generates a signal corresponding to the distribution of intensity of radiation. Through this process, the conventional optical storage reproduces information of data marks 8 recorded on recording medium 6. Conventionally, the size of aperture 3 can be smaller than the wavelength of rays of light from optical source 7. This is able to realize high-density optical storage which reads information of the data mark smaller than the wavelength of rays of light from the optical source.
However, the conventional near field optical storage must solve a tracking problem, as shown in FIG. 3. When data marks 8 are arranged in a straight line along a track 9 on recording medium 6, fiber probe 1 reads information recorded in data marks 8 of recording medium 6, moving along a specific scanning line 10. When scanning line 10 does not accord with the direction of track 9 as shown in FIG. 3, fiber probe 1 gradually deviate from track 9. Thus, the fiber probe cannot read the information recorded in data marks 8 unless the scanning line accords with the direction of track 9. Accordingly, with the conventional optical storage, it is required that track 9 and scanning line 10 accord with each other exactly.
However, since the deviation between scanning line 10 and track 9 cannot be detected by conventional techniques, the directions of track 9 and scanning line 10 should primarily accord with each other mechanically. In this case, when it is assumed that the length of the scanning line is L, allowable angle difference (a) between track 9 and scanning line 10 corresponds to the following expression. EQU La&lt;d1, that is, a&lt;d1/L
Since d1 is 50 nm and L is 1 mm conventionally, angle difference (a) becomes less than 0.003 (deg). This is difficult to realize actually. Accordingly, the aforementioned tracking problem must be solved in order to realize the optical storage using near field optical effect. The above-mentioned optical storage performs tracking using one fiber probe. Thus, it cannot read the information recorded on the recording medium because of the track direction and scanning direction out of accordance with each other. To reduce error in reading of information, the conventional optical storage has employed a method of making the scanning direction of the fiber probe accord with and track direction physically. However, this optical storage could not completely correct the error because the track and scanning line for every recording medium are difficult to accord with each other.